Patent Application
20240254577
The present disclosure relates to the technical field of a bar of alloy steel, in particular to a steel for producing a ball screw having high strength and resistance to low temperatures, and a manufacturing method thereof.
In mechanical equipment, the ball screw is an indispensable transmission element for power transmission and displacement transmission. According to the different serving environments, the ball screw for use in some extreme environments must not only have the high precision and the high wear resistance as the traditional screw, but also meet the requirements of maintaining high strength and toughness in harsh environments, such as under strong winds, huge waves, or extreme coldness in the polar regions of the earth.
The traditional ball screw uses high-carbon chromium bearing steel, such as GCr15 steel, etc. After quenching and tempering, this kind of steel can only meet the service requirements in the rigidity for contacting the steel ball, but cannot meet the service requirements in toughness at a low temperature under extreme environment. Besides, because it is difficult to control the deformation during heat treatment of the high-carbon bearing steel, the axial expansion and contraction properties of this type of material are main factors that lead to a substandard grinding accuracy of the ball screw. In addition, due to the high carbon content of this type of steel, the grinding performance after quenching is poor, and the incidence of quality problems such as grinding cracks is high.
The present disclosure provides a new kind of steel for the ball screw having high strength and resistance to low temperatures, and a manufacturing method thereof. The steel makes the processed products of ball screws have ultra-high hardness, strength and wear resistance on the surface, even under extreme low temperature conditions, and also have ultra-high toughness at low temperatures. Moreover, in the process of processing and use, the dimensional stability is good, which guarantees the working precision of the final ball screw in service.
In order to achieve the above purpose, the mechanical properties of the steel used for the ball screw in the present application meet the following levels or requirements:
The requirements for non-metallic inclusions in steel are shown in Table 1:
The mechanical properties of the steel after quenching and tempering treatment (such as oil quenching at 880° C.+ water cooling at 450° C.) are shown in Table 2.
Steel hardness: JIS G 0561 method is used to test end hardenability, J9 mm hardness≥58 HRC (at a depth of 9 mm from the surface, hardness≥58 HRC (Rockwellhardness)).
The detailed technical solution of the present disclosure to achieve the above-mentioned performances is the following:
The spheroidizing-annealed steel for the ball screw having high strength and resistance to low temperatures according to the present disclosure has the following chemical composition in mass percentage: C: 0.40-0.70%, Si: 1.20-1.80%, Mn: 1.00-1.60%, Cr: 0.80-1.20%, S: ≤0.025%, P<0.025%, Ni: 0.10-0.60%, Cu: 0.30-0.80%, Mo: 0.10-0.40%, Al≤0.05%, Ca≤0.0010%, Ti≤0.003%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, the balance is Fe and unavoidable impurities.
The design of the above chemical composition is based on the following.
C is an element necessary to ensure wear resistance. Carbon in steel improves hardness and strength by increasing the ability of martensitic transformation, thereby improving wear resistance. However, when the C content exceeds 0.77%, it will significantly increase the crack sensitivity and reduces the toughness at low temperatures. The present disclosure controls the C content to be 0.40-0.70%.
Si is a deoxidizer in the steelmaking process, and improves the hardness, strength, elastic limit and yield ratio of steel in the form of solid solution. It reduces the diffusion rate of C in ferrite, so that the carbides precipitated during tempering are not easy to aggregate, and thus improves the steel's resistance to softening during tempering. In addition, Si reduces the oxidation during frictional heating, and increases the cold-deformation hardening rate of steel, to improve the wear resistance of the material. But a too high Si content will reduce the toughness at low temperatures. The present disclosure controls the Si content to be 1.20-1.80%.
Mn is a deoxidizing element in the steelmaking process. Mn is an element effective in strengthening steel and plays a role of strengthening the solid solution. Moreover, Mn can improve the hardenability of steel and improve the hot workability of steel. Mn can eliminate the influence of S (sulfur). In smelting, Mn and S can form MnS which has a high melting point, thereby weakening and eliminating the adverse effects of S. However, when the Mn content is higher than 1.60%, it will significantly reduce the toughness of steel. The Mn content of the present disclosure is controlled to be 1.00-1.60%.
Cr is an element that can form carbides and can improve the hardenability, the wear resistance and the corrosion resistance of steel. A part of Cr in steel replaces iron to form alloy cementite, which improves the tempering stability of steel; a part of Cr dissolves into ferrite, to produce solid solution which improves the strength and hardness of ferrite. However, when the Cr content is too high, Cr combines with carbon in the steel, which easily form large carbides, and the large carbides will reduce the contact fatigue life of the steel. Based on the above analysis, the range of Cr content in the present disclosure is determined to be 0.80-1.20%.
Al is added as a deoxidizing element in the smelting process. In addition to Al reducing dissolved oxygen in molten steel, Al and N form dispersed fine aluminum nitride inclusions, so as to refine grains. However, when the Al content exceeds 0.05%, the fluidity of the molten steel decreases significantly, which increases the difficulty of casting. The range of Al content in the present disclosure is determined to be ≤0.05%.
In the steel, Ni exists in the form of solid solution. In the composition of the present disclosure, Ni can reduce the stacking fault energy and significantly improve the impact performance of the steel at a low temperature. However, a too high content of Ni will lead to a too high content of residual austenite in the steel, which reduces strength and increases cost. The range of Ni content in the present disclosure is determined to be 0.10-0.60%.
Cu element can form fine precipitates during tempering, so as to improve the strength of steel, and Cu is also beneficial to improving the corrosion resistance of steel in extreme environments. However, a too high content of Cu tends to weaken the grain boundaries and lead to cracking. The range of Cu content in the present disclosure is determined to be 0.30-0.80%.
Mo can refine the grains of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperatures. Meanwhile, it can inhibit the brittleness caused by tempering of alloy steel. However, molybdenum alloy is an expensive alloy, in order to control the cost and achieve the expected effect, the range of Mo content in the present disclosure is determined to be 0.10-0.40%.
Ca increases the number and size of point-shaped oxides in the steel. Due to the high hardness and poor plasticity of the point-shaped oxides, they do not deform when the steel is deformed, thus easily form voids at the interface, which deteriorates the performance of the steel. Meanwhile, we also consider the cost control in smelting. The range of Ca content in the present disclosure is determined to be ≤0.001%.
The way Ti harms steel is to remain in the steel in the form of titanium nitride or titanium carbonitride inclusions. This kind of inclusion is hard and angular, which seriously affects the fatigue life of the material, especially when the purity is significantly improved and the number of other oxide inclusions is small, the harm of titanium-containing inclusions is particularly prominent. Meanwhile, we also consider the cost control in smelting. The range of Ti content in the present disclosure is determined to be ≤0.003%.
The content of oxygen represents the total amount of oxide inclusions. Brittle oxide inclusions will restrict the service life of a finished product. A large number of tests have shown that reducing the oxygen content is significantly beneficial to improving the purity of steel, especially in reducing the brittle oxide inclusions in steel. Meanwhile, we also consider the cost control in smelting. The range of oxygen content in the present disclosure is determined to be ≤0.0010%.
P may cause segregation in steel during solidification, and P dissolves in ferrite, which distorts and coarsens grains, and increases cold brittleness. Meanwhile, we also consider the cost control in smelting. The range of P content in the present disclosure is determined to be ≤0.025%. S causes hot brittleness of steel and reduces the ductility and toughness of steel. Meanwhile, we also consider the cost control in smelting. The range of S content in the present disclosure is determined to be ≤0.025%.
As, Sn, Sb, Pb and other trace elements are all non-ferrous metals with low melting points. Their presence in steel will cause soft spots on the surface and uneven hardness, so they are regarded as harmful elements in steel. Meanwhile, we also consider the cost control in smelting. The ranges of the contents of these elements in the present disclosure is determined to be As≤0.04%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.
A production process of the above steel for use in the ball screw is as follows: electric furnace or converter-refining outside the furnace-vacuum degassing-continuous casting-continuous rolling-shearing or sawing-stack cooling-spheroidizing annealing-finishing-packaging for storage.
The production process mainly has the following features:
Then the intermediate billet is heated again and further rolled into the target size. The detailed heating process is: in a section of preheating, the temperature is 650-900° C., in a section of heating, the temperature is 1000-1250° C., and in a section of soaking, the temperature is 1000-1250° C. In order to ensure that the billet is fully and evenly heated, the total heating time should be more than 2 hours. The start rolling temperature in rolling is 1000° C.-1200° C., the final rolling temperature is ≥800° C. After the rolling is completed, stack cooling.
In order to ensure the stability of the dimensional accuracy of the steel when making the ball screw, it is necessary to carry out spheroidizing annealing treatment on the above steel, and use the following spheroidizing annealing process:
If the temperature of the isothermal spheroidizing is too high, the size of the ball is too large. If the isothermal spheroidizing temperature is too low, the spheroidization rate is too low, which will affect the dimensional stability of the ball screws in subsequent heat treatment processes.
After spheroidizing annealing, straightening, and flaw detection, the final product is obtained.
Compared with the prior art, the present disclosure has the following advantages:
The present disclosure is further described in detail in conjunction with embodiments.
Embodiments 1-3 give embodiments of the chemical composition and manufacturing method of the steel for the ball screw of the present disclosure respectively, and compare them with the GCr15 bearing steel which is commonly used in the market.
The chemical composition (wt %) of each embodiment is shown in the Table 3 and Table 4.
The inclusions of the steel of each embodiment are shown in Table 5.
The mechanical properties of embodiments (oil quenching at 880° C.+ water cooling at 450° C.) are shown and compared in Table 6.
The data of end hardenability of each embodiment steel is shown in Table 7.
The microstructure of the steel in each embodiment is shown in
The production process of the steel for the ball screw in each embodiment is electric furnace or converter-refining outside the furnace-VD or RH vacuum degassing-continuous casting-continuous casting billets are squared into intermediate billets-the intermediate billets are heated and rolled into products-spheroidizing annealing-finishing-packaging for storage.
In a specific smelting, we use high-quality molten iron, scrap steel and raw/auxiliary materials, and use high-quality deoxidizer and refractory materials. In the production process of electric furnace/converter, the C content at an endpoint for tapping of the three embodiments is controlled to be 0.05-0.25%, and the P content at the endpoint is controlled to be ≤0.025%. The superheat of the continuous casting is controlled to be 15-35° C.
The continuous casting billet of each embodiment is subjected to billet rolling, and the process is shown in Table 8 below.
The intermediate billet is sent to the heating furnace and rolled into the target round bar. A specific rolling process is: in the preheating section, the temperature is controlled at 650-900° C.; in the heating section, the temperature is controlled at 1000-1250° C., and in the soaking section, the temperature is controlled at 1100-1200° C.; in order to ensure that the billet is fully and evenly heated, the total heating time is 2 hours or more. The start rolling temperature is controlled at 900° C.-1100° C., and the final rolling temperature is controlled above 800° C. After rolling, it should be cooled slowly, to make the AlN particles in the steel fine, uniform, and fully separated, thereby refining the grains and preventing the steel from showing mixed crystals; after rolling is completed, stack cooling. The rolled bar is subjected to spheroidizing annealing treatment, and the process is shown in the above-mentioned three-stage spheroidizing process diagram. After spheroidizing annealing, the bar products are subjected to flaw detection and finally put into storage.
From Tables 3, 4, 5, 6, and 7, it can be seen that, compared with the traditional GCr15 bearing steel, the steel for ball screw having high strength and resistance to low temperatures in each embodiment of the present disclosure has a better level in controlling the harmful elements such as oxygen, titanium, and non-metallic elements. Especially in terms of mechanical properties, after the same quenching and tempering process, the present disclosure has significantly better performances in yield strength, tensile strength, low-temperature impact, and resistance to softening during tempering, than the traditional GCr15 bearing steel, wherein the yield strength is increased by nearly 400 MPa or more, the tensile strength is increased by 300 MPa, the low-temperature impact performance is increased by nearly 30 J, and the hardness is increased by nearly 10 HRC. The hardenability is also significantly better than the traditional GCr15 bearing steel.
Although the preferred embodiments of the present disclosure have been described in detail above, it should be clearly understood that various modifications and changes of the present disclosure will occur to those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure shall be included within the protection scope of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202111186104.2 | Oct 2021 | CN | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CN2022/120821 | Sep 2022 | WO |
| Child | 18632418 | US |
